Given the task definition and input, reply with output. Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: Susan wanted to have a birthday party.
Sent 2: She called all of her friends.
Sent 3: She has five friends.
Sent 4: Her mom said that Susan can invite them all to the party.
Sent 5: Her first friend could not go to the party because she was sick.
Sent 6: Her second friend was going out of town.
Sent 7: Her third friend was not so sure if her parents would let her.
Sent 8: The fourth friend said maybe.
Sent 9: The fifth friend could go to the party for sure.
Sent 10: Susan was a little sad.
Sent 11: On the day of the party, all five friends showed up.
Sent 12: Each friend had a present for Susan.
Sent 13: Susan was happy and sent each friend a thank you card the next week. 
Question: How many thank-you cards did Susan send?.

5.